As an apparatus for driving a rotary machine such as a compressor, there is an apparatus including an electric driving device for generating a rotational driving force and a transmission device for changing the speed of a rotational driving force generated by the electric driving device and then transmitting the speed-changed rotational driving force to the rotary machine.
Patent Document 1 discloses that a constant-speed motor and a variable-speed motor for speed change are used as the electric driving device and a planetary gear transmission device is used as the transmission device to accurately control a gear ratio. In this apparatus, it is possible to change the rotation rate of an output shaft of the transmission device connected to the rotary machine by changing the rotation rate of the variable-speed motor.